The present embodiments relate to a co-extruded multilayer structural membrane comprising at least two polymeric sheets. More particularly, the present embodiments relate to a co-extruded multilayer membrane having a cap layer adhered to a one or more support layers.
Polymeric roof sheeting is used as single ply roofing membrane for covering industrial and commercial flat roofs. Such membranes are generally applied to the roof surface in vulcanized or cured state.
Because of its outstanding weathering resistance and flexibility, cured EPDM based roof sheeting has rapidly gained acceptance. This material is often prepared by vulcanizing the composition in the presence of sulfur or sulfur containing compounds such as mercaptans or using radiation curing. Notwithstanding the usefulness of radiation curing and sulfur curing, a potential drawback of utilizing these elastomers is the lack of adhesion of EPDM, especially cured EPDM, to itself. This is a potential concern in that, in applying EPDM sheets to a roof, it is usually necessary to splice the cured EPDM sheets together along the seams. This splice or seam area is subjected to both short term and long term stresses such as those caused by roof movement, heavy winds, freeze-thaw cycling and thermal cycling. Under certain conditions, such stresses may manifest themselves in shear forces that can result in seam peel back under severe stress conditions.
In view of the foregoing, it has been necessary to use an adhesive to bond the cured EPDM sheets together. These adhesives must not only provide sufficient strength to resist the short and long term stresses described above, they must also be resistant to oxidation, hydrolysis and chemical breakdown. Adhesives that meet these requirements are difficult to produce and can be time consuming to apply to the seams of EPDM sheets, thereby increasing the overall cost of installing the waterproof membrane.
Therefore, other materials for use in roofing membrane have been investigated. Within the last decade, thermoplastic polyolefin (TPO) sheeting has come into use in the manufacture of waterproof roofing membranes. TPO membrane provides good service life, good chemical resistance and has the advantage of being recyclable. In addition, TPO membrane can be sealed along its seams without the use of an adhesive by heating the edges of the sheets to a temperature above the melt temperature of the TPO and pressing the sheets together. This technique of joining sheets of roofing membrane, known as heat welding, provides a strong seal and results in overall time and cost savings in the application of the roofing membrane.
Nevertheless, TPO roofing membrane suffers from several distinct disadvantages. First, TPO roofing generally requires scrim reinforcement embedded within the sheets to improve the flame resistance and allow manipulation of the sheets under hot air welding. Scrim is a support structure typically comprising a mesh of interwoven strands of thermoplastic. Without such scrim, the TPO often becomes too “soupy” to laminate together and may not possess sufficient mechanical properties. This scrim adds an additional cost to the TPO membrane.
Second, such membranes exhibit relatively weak bond strength with the roofing substrate. This, along with the typical ductile behavior of most TPOs prevents such membranes from supporting high wind uplift loads. In addition, present day TPO membranes are often too stiff and lacking in compliability (as indicated by the high secant modulus properties of TPO resins) due to the relatively high crystallinity of many TPOs to be easily bent to conform to the contours of a roof, and therefore are quite cumbersome to install due to this non-pliable property.
Finally, TPO layers, especially top layers, which must be loaded with various additives such as fire retardants, UV stabilizers, anti-oxidants, funcides can be expensive to produce due to the high cost of such additives.
As disclosed in published U.S. Patent Application US2001/0003625A1 to Apgar et al., it is known to produce a heat weldable roofing membrane comprised of a layer of TPE or TPO on a vulcanized EPDM sheet. Such a membrane suffers from the fact that the processing temperature of TPO and TPE and the curing temperature of EPDM are too close, thus making it difficult to seal the seam. In addition, physically bonded thermoplastic and crosslinked rubber may reject each other in the long term, resulting in delamination and compromising the effectiveness of the membrane.
Therefore, a need exists for a heat-weldable roofing membrane that retains the advantages of TPO top layers while being more pliable, easier to install and less expensive.